1 Kyros: It’s a classic trope to have the ability to ride giant eagles to cut short an arduous journey...
2 Kyros: But!... to not use them and face unnecessary hardship and danger until near the end of the adventure.
3 Mordekai: Just because something is classic doesn’t mean you need to do it all the time!!
4 Kyros: You know what else is classic, Mordekai? A fireball to the face.

This is one of the most often asked questions about The Lord of the Rings. Just do a search for "Why didn't Gandalf use the eagles to take the Ring to Mount Doom" or similar queries and dive into the multiple attempted explanations that people have made up over time:

• The Ring would have corrupted the eagles if they carried it.
• The eagles would have refused to carry The Ring.
• The eagles are too big and obvious and Sauron would have seen them.
• The eagles couldn't just drop The Ring into Mount Doom and they would have been unable to land safely and take it into the cave thingy.
• Gandalf couldn't contact the eagles at the time.
• The eagles won't deign to be used as a courier service (but still agree to go rescue Frodo and Sam).
• Nobody could willingly destroy The Ring, so Gandalf needed Gollum to follow Frodo.
• Because if the eagles helped at the beginning it wouldn't have been much of a story.